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North Carolina Department of Insurance Office of State Fire Marshal Risk Management

North Carolina Department of Insurance Office of State Fire Marshal Risk Management State Electrical Division 2017 State Electrical Code 310.16(B)(7) - Feeders in One- and Two- Family Dwelling Units. Notice:

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North Carolina Department of Insurance Office of State Fire Marshal Risk Management

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  1. North Carolina Department of Insurance Office of State Fire Marshal Risk Management State Electrical Division 2017 State Electrical Code 310.16(B)(7) - Feeders in One- and Two- Family Dwelling Units

  2. Notice: The following illustrations are approved by the State Electrical Division of the North Carolina Department of Insurance, Office of State Fire Marshal. These illustrations are intended to demonstrate but not supersede the official written interpretations found at the following web address: http://www.ncdoi.com/OSFM/State_Electrical_Division//Default.aspx?field1=State_Electrical_Code_and_Interpretations Joseph Starling Chief State Electrical Engineer & Inspector Illustrated by: Danny Thomas State Electrical Inspector

  3. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Question 1: When is a feeder allowed to utilize Table 310.15(B)(7), for a one- or two-family dwelling?

  4. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Answer 1: The language for section 310.15(B)(7) of the 2017 NEC has been modified by substituting Table 310.15(B)(7) of the 2011 NEC by NC Amendment in place of “83 percent” throughout this section. A feeder rated 100 through 400 amperes may be sized in accordance with Table 310.15(B)(7) of the 2011 NEC when the feeder supplies the entire load of the individual dwelling unit in accordance with section 310.15(B)(7)(2). The requirements of section 310.15(B)(7)(2) are typically satisfied when installing a feeder to supply power to a subpanel in the interior of a dwelling from the busbar lugs within an exterior “combo-panel” that are protected by the main breaker and supplies all loads that are part of or associated with the dwelling unit.

  5. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Answer 1: Example Table 310.15(B)(7) is allowed to be used to size these feeder conductors based on an Amendment to the 2017 NEC. Dwelling Subpanel Feeder

  6. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Question 2: Breakers have been installed in the “combo-panel ” for supplying power to other circuits that are part of or associated with the dwelling unit, and the lugs on the bottom of the busbars supply power to a feeder protected by the main breaker that serves most of the loads that are part of or associated. AC Unit for Dwelling Acme Breaker 30 amp Now that the feeder between the panels does not supply all loads that are part of or associated with the dwelling unit, is that feeder now prohibited from being sized in accordance with Table 310.15(B)(7) associated? Subpanel Feeder

  7. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Answer 2: The feeder is not granted permission to use Table 310.15(B)(7) from section 310.15(B)(7)(2) because the feeder does not supply all loads that are part of or associated with the dwelling unit. However, section 310.15(B)(7)(3) states that no feeder for the dwelling is required to have an allowable ampacity rating greater than their service-entrance conductors. Regardless of the service-entrance conductor installation either by utility, contractor, or if nothing that meets the definition of a service-entrance conductor has been installed, the largest size the service entrance that are allowed to be sized in accordance with Table 310.15(B)(7) for that installation shall be used to determine the allowable ampacity rating of the service conductors for purposes of applying this section.

  8. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Answer 2: Example The utility supplies an underground service lateral to a 200 ampere meter-panel combo on the exterior of a single family dwelling. There is a copper feeder from the busbar lugs, protected by the 200 ampere main breaker, and feeds a subpanel located in the house. All circuits for the house are supplied from the subpanel except for a heat pump and an irrigation branch circuit with both originating in the meter-panel combo outside the dwelling. The feeder from the busbar lugs, protected by the main breaker, feeds a subpanel in the house is not granted permission by section 310.15(B)(7)(2) to use Table 310.15(B)(7) because all loads associated with the dwelling house are not supplied from the feeder (the heat pump and irrigation system). According to Table 310.15(B)(16), 3/0 AWG copper is the minimum size at 75°C that the feeder would be sized if not allowed to utilize Table 310.15(B)(7).

  9. Date: June 12, 2018 2017 Electrical Code Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Answer 2: Example: (continued) Because the utility supplied the underground service lateral on a meter-panel combo, there are no service-entrance conductors per Article 100. In an overhead or non-combo installation where service-service entrance would have been present, the service-entrance conductors would be allowed to be sized in accordance with Table 310.15(B)(7). If service-entrance cable was present, it would have been allowed to be a minimum size of 2/0 AWG copper. The electrical code recognizes that there is no logical reason to require a feeder to be larger that the rating of the service-entrance conductors (present or not) or when portions of the dwelling’s load are taken off the feeder that is essentially an extension of the service-entrance conductors except the feeder possesses overcurrent protection. Therefore, the feeder from the busbar lugs protected by the main breaker, and feeds a subpanel in the house is not required to be larger than 2/0 AWG copper in accordance with section 310.15((B)(7)(3).

  10. Date: June 12, 2018 2017 Electrical Code Service Conductors Subject: Service and Feeder Conductors Sizing in One- and Two Family Dwellings Section 310.15(B)(7) Irrigation System Answer 2: Example: (continued) Acme Breaker 30 amp Acme Breaker 30 amp 310.15(B)(7)(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2). AC Unit for Dwelling Subpanel Feeder

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